1. Field of the Invention
The present invention relates to a surface profile measuring apparatus, a method of measuring a surface profile, and a method of manufacturing an optical element.
2. Description of the Related Art
Surface profile measuring apparatuses used in measuring the surface profiles of optical elements such as lenses and mirrors are known. In general, a surface profile measuring apparatus measures the profile of a target surface, which is a surface to be measured, by picking up the image of an interference pattern formed by interference between a measuring beam reflected by the target surface and a reference beam reflected by a reference surface. The surface profile measuring apparatus includes an optical system configured to split light from a light source into a measuring beam and a reference beam, an optical system configured to integrate the measuring beam and the reference beam together and to guide the integrated beam to an image pickup device, and so forth. If such optical systems have errors, the measurements obtained by the surface profile measuring apparatus may be inversely affected.
Exemplary errors of the optical systems include distortion. Distortion refers to an error exhibited as in-plane deformity of an image formed on the image pickup surface of an image pickup device. An optical system having distortion forms a deformed interference pattern on the image pickup surface. The distortion further distorts measurements obtained by the surface profile measuring apparatus. Consequently, when the target surface is processed on the basis of the measurements, deviation may occur between the measuring coordinates and the processing coordinates. Theoretically, it is possible to design the optical systems of the surface profile measuring apparatus without distortions. To do so, however, stringent design restrictions are imposed. Such restrictions, for example, increase the number of lenses included in the optical systems, and make the surface profile measuring apparatus larger and more complicated.
Japanese Patent Applications Laid-Open No. 2000-97663 (JP 2000-97663) and No. 2002-333305 (JP 2002-333305) propose to address the above-discussed issues and disclose surface profile measuring apparatuses capable of obtaining precise measurements even if optical systems thereof have distortions. In the apparatuses disclosed by JP 2000-97663 and JP 2002-333305, the distortions of the optical systems are calculated in advance, and measurements obtained by the apparatuses are corrected on the basis of the calculated distortions, whereby precise measurements are purportedly obtained. Specifically, in the apparatus disclosed by JP 2000-97663, the distortion is calculated by performing alignment of the target surface with some deviation while the interference pattern is being measured. In the apparatus disclosed by JP 2002-333305, the distortion is calculated by measuring a primary standard as a target object in advance.
In the apparatus disclosed by JP 2000-97663, however, if the amount of driving performed for deviating the alignment of the target surface includes an error, it is difficult to separate the change in the interference pattern due to the deviation in the alignment and the change in the interference pattern due to the distortion. The amount of driving performed for deviating the alignment needs to be controlled in nanometer order, and it is therefore difficult to control the amount of driving without errors. Accordingly, with the apparatus disclosed by JP 2000-97663, it is difficult to precisely calculate the distortion.
In the apparatus disclosed by JP 2002-333305, every time the desired profile of the target surface is changed, the primary standard needs to be replaced with another one whose surface profile is the same as that of the target surface. Furthermore, a mark to be provided on the primary standard needs to be formed on the basis of in-plane coordinates whose precision is not guaranteed. Accordingly, from the foregoing, it is evident that it is not easy to calculate the distortion.